Sunn Beta Bass Power

[Rob Bagg]

I'm curious about how much power output I'm getting at 8ohms with my Beta Bass Head. I know it's rated at 100watts into 4 ohms. Is there a formula to figure the output with different loads?

[Don T.]

Hello:
 RMS Volts/resistance in OHMS = AMPS
 Amps*RMS Volts = Watts

Given the voltage does not change, doubling the resistance will half the power. However this is rarely true because the power transformer voltage will sag when the load is increased. SO......... 100 watts into 4 ohms will most likely net around 60~65 watts into 8 ohms.

Example: SA21 nets 300 watts (per channel, both sides driven) @ 2ohms
                              200 watts (per channel, both sides driven) @ 4 ohms
                              125 watts (per channel, both sides driven) @ 8 ohms

[JoeArthur]

The other formula, which I prefer to use because it's somewhat easier
for my purposes is the signal voltage squared divided by the load:

20 volts * 20 volts = 400 / 4 = 100 watts

Which as Don pointed out, the same output voltage will result in half
the power when the load reactance is doubled:

20 volts * 20 volts = 400 / 8 = 50 watts

The 60-65 watts at double the load reactance (i.e. a lower load) is a rule
of thumb.  Certain power amp designs might not be able to increase
the voltage swing across a lighter load.  Meaning, that at some load
value, the output stage can be limited in the voltage it can swing by
the supply voltage value and its design.  

Some may also be limited by "protection circuitry" - especially those circuits that sense the output voltage to perform their protection duties.